Explicitly generate Fortran callback signatures using %fortrancallback.
Function pointer wrapper code now uses $*fclassname to determine a specific callback type that corresponds to the function signature, rather than defaulting to the opaque SWIGTYPE*.
If the callback function signature isn't defined, it will create a new abstract interface.
%fortrancallback.$*fclassnameto determine a specific callback type that corresponds to the function signature, rather than defaulting to the opaqueSWIGTYPE*.Closes #116 .